1. Field of the Invention
The present invention concerns a method for the fabrication of an X-ray image intensifier tube. It also concerns X-ray image intensifier tubes with high vacuum obtained thereby.
2. Description of the Prior Art
X-ray image intensifier tubes are well known in the prior art. They convert an X-ray image into a visible image, for example in order to provide for medical observation.
The problem that arises, and that the present invention seeks to resolve, is the fact that a spurious illumination is observed in the X-ray image intensifiers, even when there is no X-radiation. This spurious illumination is troublesome for the observation screen. It is due to the alkaline metals deposited involontarily on the electrodes of the X-ray image intensifier tubes when the photocathode is being made. The intense electrical field that prevails in the tube manages to liberate electrons from these alkaline metals which are highly electropositive and, therefore, easily ionizable. These electrons go up along the electrical field, strike the observation screen and create a spurious illumination.
It will be recalled that alkaline antimonide type photocathodes are fabricated in the vacuum chamber of the X-ray image intensifier tube, for alkaline metals are higly reactive and have to be created under vacuum to be stable. These photocathodes can be made by successive operations for the vapor deposition of their constituent elements, in the tube, using a common type of crucible containing antimony, the evaporation of which is caused by heating the crucible, by Joule effect for example. The alkaline metals are evaporated by means of generators generally located on the electrode closest to the anode.
The evaporation of alkaline metals is the result of a silicothermic or aluminothermic process on the chromates or metals sought to be evaporated. The silicothermic or aluminothermic processes are set off by the heating, by Joule effect, of the alkaline generators.
Alkaline generators are far less directive than antimony generators. This is because, for the silicothermic or aluminothermic processes to take place efficiently, it is necessary to use special crucibles in which the chromates are confined. This type of crucible has poor directivity, the advantage of which is that it ensures a properly uniform deposition of the alkaline metals throughout the surface of the photocathodes which is at a distance from these crucibles. On the other hand, it has the drawback of causing the deposition of alkaline metals on all the parts of the X-ray image intensifier tube, and notably on the electrodes, thus causing the problem of spurious illumination on the observation screen.
To resolve this problem, one method used by the Applicant is to put a coating of aluminium oxide AlO.sub.3 on the electrode closest to the anode, generally made of aluminium itself.
This approach makes it possible to eliminate the spurious illumination of the observation screen but introduces electrical discharges through this layer of oxide, which is an electrical insulator.
When the X-ray image intensifier tube receives an X-radiation, a portion of the electrons coming from the photocathode falls on this electrode. Since this electrode is coated with a layer of oxide, these electrons do not flow and discharges are created through the oxide layer.
In another known approach to the problem referred to, which does not have the drawbacks of the above known approach, before introducing it into the intensifier there is deposited, at least on a part of an electrode or electrodes, a layer of an electricity conducting material having the property of oxidizing the alkaline metals that go into the composition of the photocathode. This material may be chosen from among the following elements: Te, Se, S, P, in preferred embodiments.
The present invention seeks to propose a variant of this latter approach which preserves the advantages of a barrier layer holding the electrons of the conductive alkaline metals while, at the same time, making its application and implementation simpler.